1. Field of the Invention
This invention relates generally to an internal combustion engine and more particularly to such an engine incorporating tensioner means for automatically tensing a cam chain or the like which operatively connects a crankshaft and a camshaft together for operating intake and exhaust valves.
2. Prior Art
Japanese patent application Laid-Open (Kokai) No. 59-208250 discloses one conventional tensioner for keeping a cam chain or timing chain taut, the tensioner comprising a hydraulic lock mechanism for urging a tensioner member into sliding contact with an outer periphery of the endless cam chain which is extended around and engaged with a first sprocket on a crankshaft and a second sprocket on a camshaft. The hydraulic lock mechanism allows the tensioner member to be moved toward the cam chain but prevents it from being moved away from the cam chain under the influence of hydraulic pressure. The tensioner comprises a mounting member for mounting the hydraulic lock mechanism on an engine body, the mounting member having an oil reservoir from which oil is fed to the hydraulic lock mechanism. More specifically, the mounting member of a one-piece cast construction has an upwardly-opening well, serving as the oil reservoir, and a vertically-disposed bore disposed adjacent to the well. The hydraulic lock mechanism comprises a cylinder received in the bore and a hollow plunger received in the cylinder for sliding movement therealong. A hydraulic chamber is formed at a lower portion of the cylinder extending between the bottom of the plunger and the bottom of the cylinder. The oil is fed from the oil reservoir to the hydraulic chamber via a passage extending through the peripheral walls of the cylinder and plunger, the interior of the hollow plunger and a valve port formed through the bottom of the plunger. A spring is received within the hydraulic chamber and acts on the bottom of the plunger to urge it upwardly. A check valve element in the form of a ball is also received in the hydraulic chamber and normally closes the valve port under the influence of the hydraulic pressure in the hydraulic chamber. An upper end of the plunger is urged against one arm of a pivotal bell-crank lever under the influence of the spring while the other arm of this lever is urged against the tensioner member to keep the cam chain taut. When the cam chain becomes loose, the plunger is moved upwardly, so that the pressure in the hydraulic chamber is reduced to create a pressure differential between the interior of the hollow plunger and the hydraulic chamber. As a result, the check valve element is moved away from the valve port to supply the oil to the hydraulic chamber from the oil reservoir. On the other hand, when the cam chain is subjected to an undue pulling force, the plunger tends to be moved downwardly. However, this downward movement is prevented by the check valve element.
The mounting member is secured to the engine body by bolts at upper and lower portions thereof. Lubricating oil is caused to drop into the upwardly-opening oil reservoir after it lubricates the second sprocket mounted on the cam shaft. Since the mounting member is cast of metal, the wall thickness thereof is relatively large, and therefore the upwardlydirected opening of the oil reservoir is of a relatively small size. Therefore, there is the possibilities that the oil is not sufficiently fed to the hydraulic lock mechanism. It is considered to form the mounting member by processing a metal sheet. However, the oil reservoir or well is subjected to deformation when the mounting member is secured to the engine body by bolts. The deformation of the mounting member also causes the deformation of the hydraulic lock mechanism. In addition, although the hydraulic lock mechanism and the mounting member are disposed within a closed path of travel of the cam chain, the bell-crank lever is disposed outside this closed path of travel of the cam chain, which increases the size of the engine